Gas turbine engines have known a long and successful use in the aircraft industry. More commonly known as jet engines, the gas turbine has made high speed air travel relatively safe and convenient. Air travel is still costly, however, so manufacturers are continuously striving to reduce both manufacturing and operating costs OF the aircraft gas turbine engine.
One of the origins of increased operating cost is leakage of air and other gases through various parts of the engine without extraction of the energy contained in that air and gas. Rotating labyrinth seals have been frequently used in the past to close such leakage gaps. Another candidate for doing so is a brush seal.
A basic brush seal includes a front or upstream plate, a backing or downstream plate, and a bristle pack sandwiched between the two annular plates. The bristle pack extends across a leakage gap that exists between two relatively movable members so that the free ends of the pack sealingly engage the surface of one of the members. The bristle pack creates a torturous flow path for any air or gas leaking from one side of the seal to the other. Usually, one of the movable members is a rotating structure such as a shaft and the other member is a stationary structure of the engine.
The gap between the backing plate and the rotating engine member is usually sized so as to be large enough to accommodate expected operating transient conditions. Since the size of the leakage gap is related to the ability of the seal to perform its intended sealing function, increasing the gap to accommodate engine operating transients affects the ability of the seal to operate. That is, it can be shown that the maximum pressure difference from the upstream side to the downstream side of the seal is an inverse function of the backing plate gap. Thus, as the gap becomes larger, its ability to function effectively declines accordingly. To prevent damage to the seal or to the opposing rotor member during transient conditions, however, the leakage gap must be of a certain minimum size. This minimum size requirement may result in making the brush seal ineffective for a particular desired application even though its use would otherwise be preferred.
Thus, it would be desirable to have a brush seal that was more tolerant of gas turbine engine operating transients so as to be able to experience such transient operating condiditons without damage while maintaining at all other times the desired clearance gap and desired pressure differential between the upstream and downstream sides of the seal.